1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for optically recording or reproducing information and, more particularly, to an optical information recording/reproducing apparatus using a semiconductor laser oscillator as a light source.
2. Description of the Related Art
Various types of optical information recording/reproducing apparatuses are developed. In such an optical information recording/reproducing apparatus, a laser light is conventionally obtained from a semiconductor laser oscillator arranged in an optical head, and information is recorded to an information recording medium (optical disk) using the laser light, or the recorded information is read out (reproduced or played back) from the optical disk.
Normally, an optical information recording/reproducing apparatus is provided with an optical pickup using a semiconductor laser oscillator. In such an optical pickup, a few % of the laser light reflected at the surface of an optical disk is fed back to the semiconductor laser oscillator. This fed-back reflection increases the noise level of the laser oscillator.
For an apparatus designed to accurately recording/reproducing information to/from the optical disk, the above-mentioned increased noise of the laser oscillator is an important problem to be solved. For instance, increased noise of the laser oscillator degrades the tone quality of a PCM (Pulse Code Modulation) audio player or a CD (Compact Disk) player and also degrades the image quality of a laser video disk player.
The main factor of the noise of a semiconductor laser oscillator resides in a fact that part of laser light reflected at the mirror surface of an optical disk is returned to the semiconductor laser oscillator, which factor causes a return-light-induction noise. More specifically, the return light is mixed into the light in the laser resonator so as to cause a coupling resonance effect, thereby making the laser oscillation mode unstable.
There is a countermeasure to the above unstable laser oscillation. That is, a high frequency signal superimposed to a laser driving signal so that the laser oscillator oscillates in a multi-longitudinal mode oscillation, thereby lessening the influence of the return light.
For instance, according to a prior art method disclosed in Japanese Patent Disclosure (kokai) No. 59-9086 or U.S. Pat. No. 4,480,325 (Aiki et al. ), a high frequency oscillation output current is superimposed to a drive current of the semiconductor laser oscillator in order to obtain a laser light output in the multi-longitudinal mode, thereby suppressing the noise in the laser light output.
In the above prior art method, a sine oscillation waveform is normally adapted to the superimposed high frequency current. The optimum values of the intensity and the frequency of this high frequency current depend on the type of the semiconductor laser oscillator. Further, even if the optimum intensity and optimum frequency are adapted, a highly stable and low noise laser oscillation cannot be satisfactorily obtained.
Thus, according to the above-mentioned prior art semiconductor laser oscillator using a multi-longitudinal mode oscillation, it is difficult to achieve a highly stable and low noise laser oscillation.